An inverter-driven rotating electrical system that drives a rotating electrical machine using an inverter is now developed and widely used. In this inverter-driven system, the inverter transforms a DC voltage into a pulse voltage through switching operation and supplies the pulse voltage to the rotating electrical machine through cables. The rotating electrical machine is driven by a power of this pulse voltage.
Conventionally, in a high-voltage rotating electrical machine, an electric field reduction system is often applied to a surface of a coil provided around a stator core end portion in order to prevent occurrence of a partial discharge or heat generation, especially, around a core end portion of a stator coil. In the electric field reduction system, a low resistance layer led from an inside of a stator core slot and an electric field reducing layer formed so as to overlap a part of the low resistance layer are combined.
On the other hand, in the inverter-driven system, a reflected wave may occur by impedance mismatching among the inverter, the cables, and the rotating electrical machine. If the reflected wave is superimposed on the pulse voltage, high-voltage noise, so-called inverter surge may occur between the cables and the rotating electrical machine, especially, at a connecting portion between the cables and the rotating electrical machine.
When the pulse voltage including the inverter surge (hereinafter, referred to as “inverter pulse voltage”) repeatedly occurs, the partial discharge and heat generation at the stator coil of the core end portion (hereinafter, referred to as “stator coil end”) which are caused during operating time by commercial frequency become more serious. And the partial discharge and heat generation also cause on the electric field reduction system. As a result, the reliability of the stator coil may be significantly affected.
The partial discharge and heat generation depend on a surface potential of the electric field reduction system (see Non-patent Document 1). Thus, there has been a strong need for a technology that accurately measures the surface potential of the electric field reduction system in which the inverter pulse voltage is generated.